The invention relates to a device for handling ball heads of prosthetic joints.
Prosthetic joints in which one joint element is in the form of a socket and the other joint element is in the form of a ball head which is mounted rotatably in the socket are known especially as shoulder and hip prosthetic joints. Such prostheses are usually of modular design. Hip endo-prosthetic joints, for example, consist of the socket, which is inserted into the hip bone, and the shank, which is inserted into the thigh bone. The shank has a pin, the so-called cone, onto which the ball head is pushed. Especially in that modularly designed endo-prosthesis, implant components of various materials and sizes are combined with one another in order to provide adaptation to the physical build of the patient. For example, ball heads made of a cobalt/chromium alloy or of an aluminium oxide ceramics material are pushed onto a cone made of a titaniumr or cobalt/chromium/molybdenum alloy. The technique used for connecting metal or ceramics ball heads to the cone is press-fit connection, especially a conical press fit. In that technique, the ball head, which has a conical bore, is placed on the cone. Once the ball head has been pushed onto the cone, it is fixed in place by an impact to the ball head.
As is known from the publication xe2x80x9cDas Prinzip der Konus-Steckverbindung fxc3xcr keramische Kugelkxc3x6pfe bei Hxc3xcftendoprothesenxe2x80x9d [The principle of the conical socket connection for ceramics ball heads in hip endo-prostheses] by G. Willmann, Mat.-wiss. U. Werkstofftech. 24, pages 315-319 (1993), all ceramics materials are brittle. In the case of loads that are not applied over an area and in the event of damage to the surface as a result of scratches, concentrations of stress may occur, resulting in the destruction of the ball head. In metal ball heads, scratches may occur which may in turn cause friction in the surrounding socket.
In order to protect the ball heads from damage until they are used in the operating theatre, it is known to provide them with a protective cap that is matched to the shape of the ball. In the case of ball heads that differ only slightly in diameter, protective caps having the same internal diameter are used. The protective caps are usually removed only in the operating theatre. In its lower region, a protective cap is divided into blades which engage around the ball head to below its equatorial diameter and thus automatically hold the protective cap on the ball head. The ball heads of hip endo-prosthetic joints are positioned on the shank by the surgeon by hand. For that purpose, the protective cap must be removed, which is difficult in the vicinity of the surgical wound because the surgeon""s gloves will be wet with bodily fluids. As the ball head is being positioned, its polished surface may become scratched by residues on the gloves. In the case of metal ball heads, this can result in increased friction in the socket. Moreover, there is an additional risk of damaging the surface of the ball heads because of the large number of instruments that are used during an operation. A further difficulty during the operation lies in reliably identifying ball heads that differ only very slightly in diameter.
The problem underlying the present invention is therefore to construct the protective caps so that handling of the ball heads during the operation is facilitated, protection of the ball heads is provided also while the shank is being pushed onto the cone and the possibility of distinguishing between different sizes and materials is improved.
The problem is solved by means of a device for handling ball heads of prosthetic joints, in which the surface of each ball head is covered by a protective cap to beyond the equatorial diameter before being positioned on the cone of a shank, so that the protective cap is held automatically on the ball head and can be removed without a tool, characterized in that there is arranged on the pole of the protective cap a crown in the form of a plate, and the shape of the plate and its extend relative to the protective cap are such that it is possible to grip behind the plate on the side facing the protective cap.
On the pole of the protective cap, which is intended to protect the ball head against damage from the time of its manufacture until it is to be used, the device according to the invention for handling ball heads has a crown in the form of a plate. The shape of the plate and its extent relative to the protective cap are such that it is possible to grip behind the plate on the side facing the protective cap. The plate may be round or polygonal, or may alternatively have a different circumferential shape.
Whilst it is very difficult during the operation for a surgeon to remove a conventional protective cap on a ball head with gloves that are wet with bodily fluids and whilst the ball head is difficult to handle because of its polished surface, the device according to the invention for handling ball heads allows the ball head to be held securely until it is positioned on the shank and to be guided reliably. The crown allows the ball head to be positioned by its conical opening onto the cone of the shank in a targeted manner. The surface on the plate of the crown may additionally serve as an impact surface for the fixing tool with which the ball head is pressed onto the cone by an impact. The plate offers a larger surface area for the impact tool to strike than does the curved surface of a ball. The sensitive, polished surface of the ball is also protected against damage caused by the impact tool.
During the entire handling operation until fixing on the cone of the shank has been effected, the surface of the ball head is constantly protected against damage caused by surgical tools or by contamination. Once the ball head has been fixed on the cone of the shank, the protective cap can be removed easily from the ball head by means of the crown that can be gripped from behind. During removal, the blades of the protective cap expand and release the ball head. The material of the device can be, for example, a tough impact-resistant plastics material.
In an advantageous development of the invention, the plate is so arranged that it lies perpendicular to the connecting line extending from the centre point of the ball head to the pole of the protective cap. This enables the device for handling ball heads to be aligned with the cone of the shank. The surface of the plate may be flat or slightly convexly curved. As a result, it can also serve as an impact surface for the fixing tool.
In a further advantageous development of the invention, a neck may be arranged between the protective cap and the plate of the crown. Such a neck, which may be, for example, about 1 cm in length, further facilitates the possibility of gripping the plate from behind and, thus, handling. The neck also provides the possibility of specifically directing the fixing force to the ball. As the ball head is positioned on the cone of the shank, the force applied by the fixing impact is directed precisely in the longitudinal direction of the cone when the neck is aligned precisely in the longitudinal direction of the cone.
The neck may also extend in the form of a rod and may be, for example, up to 100 mm in length. A device for handling ball heads of such construction can be used, for example, for ball heads that have not been delivered already covered with a protective cap. In preparation for the operation, those ball heads can be inserted into such a handling device. The rod then acts as an alignment and insertion aid for the surgeon. Because of the length of the rod, that device provides very good scope for positioning and placing on the shank.
A ball head can be positioned easily on the cone of a shank when the centre line of the conical opening of the ball head is so aligned that it extends through the pole of the protective cap of the handling device according to the invention. When the crown has a neck or a rod for the plate, these may be aligned in the direction of the cone. The cone and the opening in the ball head are thus in alignment with one another and the ball head needs only to be pushed onto the cone.
In a further development of the invention, the diameter of the plate may have a particular relationship with the diameter of the ball head in question. As a result, it is possible to give an indication of the diameter of the ball head by virtue of a plate diameter that has been determined beforehand relative to the diameter of the ball head. It is thus considerably easier for the surgeon to distinguish between ball heads of different diameters during the operation.
The possibility of distinguishing between ball heads can be increased still further in a further development of the invention wherein the surface of the plate that is remote from the protective cap bears an identification marker. That identification marker may be written or engraved on, or may be a coloured identification marker. The identification marker can provide information, for example, relating to the material of the ball head, its diameter, the type and size of its conical bore and the batch data and manufacturer. The surgeon would thus be better and more reliably able to distinguish between ball heads.
The invention will be explained in greater detail with reference to embodiments.